The present invention relates to a distance measurement apparatus and a distance measuring method. More particularly, it relates to a distance measurement apparatus and a distance measuring method in which a light flight time is detected and a distance from an object is detected, so that precision of distance measurement is enhanced.
Known is a conventional distance measuring technique by a time-of-flight system for detecting a timing of a reflected light which is reflected and returned by an object, detecting a time difference from a light emitting timing, and obtaining a light flight time to detect a distance to the object.
Moreover, another distance measuring method is usually a triangular survey system. As compared with the triangular survey system, in the time-of-flight system, there are advantages that the distance can be detected from one observation point and the apparatus can therefore be miniaturized, and that any occlusion as a problem of triangular survey does not occur and the distances to all object points from the observation point can therefore be detected.
Moreover, there are some methods for measuring a delay of the light reflected by the object. For example, in a method in which a charge distributing detector is utilized as disclosed in xe2x80x9cIntegration-Time Based Computational Image Sensorsxe2x80x9d, 1995 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors, Apr. 20 to 22, 1995, Dana Point, Calif., USA, a distance extraction processing can be simplified. Additionally, since a semiconductor manufacturing technique is used, the method is suitable for miniaturizing the apparatus.
A constitution of the charge distributing detector disclosed in xe2x80x9cIntegration-Time Based Computational Image Sensorsxe2x80x9d, 1995 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors, Apr. 20 to 22, 1995, Dana Point, Calif., USA, and the distance measuring method utilizing the charge distributing detector will next be described.
FIGS. 8A, 8B show a basic constitution of the charge distributing detector and a principle of a charge distributing operation by the detector.
First, the constitution of the charge distributing detector will be described.
That is to say, as shown in FIG. 8A, the charge distributing detector is constituted of an MOS light receiver 101 formed on a P-type semiconductor substrate 100, a first transfer gate 103 disposed in the vicinity of the MOS light receiver 101 and between the MOS light receiver and a first charge accumulation area 102, and a second transfer gate 105 disposed in the vicinity of the MOS light receiver 101 and between the MOS light receiver and a second charge accumulation area 104.
The MOS light receiver 101 is provided with a gate electrode having a good light transmittance, and a voltage is applied to the gate electrode to form a depletion layer on the surface of the P-type semiconductor substrate 100.
On the other hand, an electric potential is applied to the first and second charge accumulation areas 102 and 104 to form a potential well deeper than the semiconductor surface potential of the MOS light receiver 101.
Moreover, transfer pulses "PHgr"WG1 and "PHgr"WG2 can individually be applied to the first and second transfer gates 103 and 105.
A charge distributing operation by the charge distributing detector constituted as described above will next be described.
First, while the second transfer gate 105 is closed, a transfer pulse is applied to "PHgr"WG1 to turn ON the first transfer gate 103. Then, as shown by a solid-state line in FIG. 8B, electrons photo-generated in the MOS light receiver 101 flow into the first charge accumulation area 102 via the first transfer gate 103, and are accumulated as an electric charge Q1 in the first charge accumulation area 102.
Conversely, while the first transfer gate 103 is closed, the transfer pulse is applied to "PHgr"WG2 to turn ON the second transfer gate 105. Then, as shown by a broken line in FIG. 8B, the electron photo-generated in the MOS light receiver 101 flows into the second charge accumulation area 104 via the second transfer gate 105, and is accumulated as an electric charge Q2 in the second charge accumulation area 104.
When the transfer pulses "PHgr"WG1 and "PHgr"WG2 applied to the first and second transfer gates 103 and 105 are alternately turned ON in this manner, it is possible to control a transfer direction of the electron photo-generated in the first and second charge accumulation areas 102 and 104.
The distance measuring principle using the charge distributing detector will next be described with reference to FIGS. 9 and 10A to 10D.
As shown in FIG. 9, a measurement system is constituted of a pulse generating light source 110, charge distributing detector 111 and timing controller 112.
Here, the timing controller 112 establishes synchronization in the pulse generating light source 110 and charge distributing detector 111, and further controls an operation timing.
It is assumed that a distance to the object 113 from the measurement system is R. For convenience of description, it is assumed that the distance to the object 113 from the pulse generating light source 110 is the same as the distance to the object 113 from the charge distributing detector 111.
Moreover, as shown in FIGS. 10A to 10D, first, the object 113 is irradiated with a pulse light with a light emitting time T from the pulse generating light source 110.
The light reflected by the object 113 reciprocates by the distance R, that is, flies by a distance 2R, and is incident upon the charge distributing detector 111 with a delay of xcex94t=2R/c (here, c indicates light speed) behind the pulse light emitting timing.
In this case, the transfer pulse "PHgr"WG1 is applied to the first transfer gate 103 of the charge distributing detector 111 at the same timing as that of the light emitting pulse, and the transfer pulse "PHgr"WG2 is applied to the second transfer gate 105 at a timing at which the light emitting pulse turns off. During this, the photo-generated electron generated by the reflected light is detected.
Here, it is supposed that there is no cross talk in an electric charge transfer operation and that electric charge transfer is switched at a sufficiently high speed. Then, the electric charge Q1 accumulated in the first charge accumulation area 102 and electric charge Q2 accumulated in the second charge accumulation area 104 are represented by the following equations (1) and (2), respectively.
Q1=xe2x88x92Qtxc3x97(Txe2x88x92xcex94t)/T xe2x80x83xe2x80x83(1) 
Q2=xe2x88x92Qtxc3x97xcex94t/T xe2x80x83xe2x80x83(2) 
Therefore, the distance R to the object can be obtained by the following equation (3).
R=(cT/4)xc3x97{(Q1xe2x88x92Q2)/(Qtxe2x88x921)}xe2x80x83xe2x80x83(3) 
As described above, in the distance measuring method in which the charge distributing detector is used as described above, the distance to the object can be measured with a very simple processing. As compared with the distance measuring method which is based on the triangular survey method, the distance to the object can be detected only by observation from one point. Therefore, it is possible to miniaturize the apparatus and to realize a very superior distance measurement apparatus in which no occlusion occurs.
Moreover, the charge distributing detector can be prepared by applying a technique of a charge transfer device (CCD) frequently used in a solid-state image pickup device, and is advantageous for integration of the apparatus.
Additionally, a shot noise by light quantum fluctuation exists in the signal charges Q1 and Q2.
A standard deviation amount of the shot noise is generally a square root of the number of incident photons. However, since the photon having an energy of a visible light area usually generates a single electron-hole pair in a semiconductor, it is unnecessary to consider an amplification fluctuation. Additionally, a process of exciting the electron in the semiconductor is also a quantum effect, and it may be supposed that each electron exciting process has no correlation among electrons. Therefore, the shot noise proportional to the square root of the number of electrons exists in the final signal charges Q1 and Q2.
That is to say, Q1 includes noise (qxc2x7Q1)xc2xd and Q2 includes (qxc2x7Q2)xc2xd (here, q denotes an elementary electric charge).
When the shot noise is considered, and a noise term is considered in the equation (3) by first approximation, the following equation (4) results.
R=(cT/4)xc3x97{(Q1xe2x88x92Q2)/(Qt)+[SQR(q/Qt)]}xe2x80x83xe2x80x83(4) 
Here, a first term with parentheses 1 denotes a signal component, and a second term denotes a noise component.
Moreover, Qt=Q1+Q2. An underlined portion denotes an average value, and a value in brackets indicates a standard deviation value of an alternating noise component.
As seen from the second term, the noise of a conventional charge distributing pixel does not depend on charge distributing states of Q1 and Q2, and depends only on a total signal charge amount.
Signal deterioration by the shot noise can be suppressed by securing a sufficient light amount. However, when the apparatus is reduced in size and power consumption, the devices are two-dimensionally arranged, and multiple points are simultaneously measured, there is a practical limitation in the light emitting amount of the pulse generating light source, and it is required to secure precision even with a smaller light amount.
The present invention has been developed in consideration of the aforementioned situations, and an object thereof is to provide a distance measurement apparatus using a charge distributing detector which can obtain a satisfactory distance measurement precision even with a smaller light amount.
Another object of the present invention is to provide a distance measuring method using a charge distributing detector which can suppress signal deterioration by a shot noise and obtain a satisfactory distance measurement precision with a smaller light amount.
To achieve the aforementioned objects, according to one aspect of the present invention, there is provided a distance measurement apparatus for irradiating an object with a light from a light source whose luminance can be modulated or from a pulse light source, and receiving the reflected and returned light to obtain a distance to the object, the distance measurement apparatus comprising:
a photoelectric converter for receiving the reflected light and photoelectrically converting the received light;
a first charge accumulator for accumulating an electric charge transferred via a first gate driven by a first transfer pulse synchronized with an emitting timing of the light from the light source among electric charges generated by the photoelectric converter;
a second charge accumulator for accumulating an electric charge transferred via a second gate driven by a second transfer pulse complementary to the first transfer pulse among the electric charges generated by the photoelectric converter; and
a normalization circuit for reading a first signal based on the accumulated electric charge of the first charge accumulator, and a second signal based on the accumulated electric charge of the second charge accumulator, and normalizing the smaller signal of the first signal and the second signal with an added signal of the first signal and the second signal.
Moreover, according to another aspect of the present invention, there is provided a distance measurement apparatus for irradiating an object with a light from a light source whose luminance can be modulated or from a pulse light source, and forming an image of the reflected and returned light on a two-dimensional pixel array by an optical system to obtain a distance to the object for each pixel, the distance measurement apparatus comprising:
a two-dimensional pixel array in which each pixel comprises a photoelectric converter, a first gate for transferring an electric charge generated by the photoelectric converter to a first charge accumulator, and a second gate for transferring the electric charge generated by the photoelectric converter to a second charge accumulator, and control terminals of the first gate and the second gate of each pixel are connected in common;
a line selection circuit for selecting a line of the two-dimensional pixel array;
a line parallel reader for reading a first signal based on the electric charge accumulated in the first charge accumulator and a second signal based on the electric charge accumulated in the second charge accumulator in parallel with respect to the pixel of the line selected by the line selection circuit;
a normalization circuit for normalizing the smaller signal of the first signal and the second signal read by the line parallel reader with an added signal of the first signal and the second signal for each row of the two-dimensional pixel array;
a first transfer pulse applicator for applying a transfer pulse synchronous with an emitting timing of the light from the light source to the first gate; and
a second transfer pulse applicator for applying a transfer pulse complementary to the transfer pulse applied to the first gate to the second gate.
Furthermore, according to further aspect of the present invention, there is provided a distance measuring method for irradiating an object with a light from a light source whose luminance can be modulated or from a pulse light source, and receiving the reflected and returned light to obtain a distance to the object, the method comprising the steps of:
receiving the reflected light and photoelectrically converting the received light;
accumulating an electric charge transferred via a first gate driven by a first transfer pulse synchronous with an emitting timing of the light from the light source among electric charges generated by the photoelectric converter in a first charge accumulator;
accumulating an electric charge transferred via a second gate driven by a second transfer pulse complementary to the first transfer pulse among the electric charges generated by the photoelectric converter in a second charge accumulator; and
reading a first signal based on the accumulated electric charge of the first charge accumulator, and a second signal based on the accumulated electric charge of the second charge accumulator, and normalizing the smaller signal of the first signal and the second signal with an added signal of the first signal and the second signal.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.